Control of print quality in a printed matter (e.g., ink color adjustment) is performed by measuring a reflection property such as a print density or color values of ink on a CMYK (Cyan, Magenta, Yellow, Black) test patch. For example, a print density of ink is measured according to ISO 5-4. In this measurement, a so-called 45/0 geometry in which illumination light is emitted in a direction at 45° with respect to a normal line to a sample surface (in a 45-degree direction), and, among reflected light rays reflected by the sample surface, a reflected light ray in the normal line is received, or a so-called 0/45 geometry in which the illuminating direction and the light-receiving direction are interchanged with each other.
In order to quickly respond to a change in ink concentration during printing to thereby suppress the occurrence of a defective product, it is necessary to measure a print density of ink just after printing. However, when ink in an undried state just after printing is dried, the print density changes due to dry-down. Thus, the print density of the ink just after printing cannot be directly compared to a density of a reference sample which is in a dried state. The term “dry-down” here means a phenomenon that, along with drying of ink on a printing matter, a print density of the ink after being dried drops down, as compared to a print density of the ink just after printing. This dry-down is caused by a difference between reflection of light on a surface of an ink layer just after printing (a layer of ink formed on a sheet surface by printing), and reflection of the light on a surface of the ink layer in a dried state of the ink. More specifically, in the state just after printing, the ink layer has a flat and smooth surface, irrespective of irregularities of the sheet surface, so that illumination light emitted in a 45-degree direction is specularly or regularly reflected by the flat and smooth surface of the ink layer, and the reflected illumination light does not enter a light-receiving system capable of receiving a reflected light ray in a 0-degree direction (direction of a normal line to the sheet surface). On the other hand, in the state after the ink is dried, the ink layer has a surface conforming to irregularities of the sheet surface (i.e., a surface with a concavo-convex shape), so that the illumination light is scatteredly or irregularly reflected by the concavo-convex surface of the ink layer, and a part of the reflected illumination light enters the light-receiving system. This causes the dry-down.
Therefore, in a conventional density measurement, two polarizing plates whose polarizing directions (polarizing axes, polarization properties) mutually intersect orthogonally are inserted, respectively, into an illuminating optical system and a light-receiving optical system, and, in this state, the print density is measured. This eliminates an influence of the dry-down to thereby allow a print density of ink just after printing to be effectively compared with the density of the reference sample. In this density measurement, by utilizing a phenomenon that a polarization characteristic of illumination light after being polarized by the polarizing plate inserted in the illumination optical system is maintained even in the illumination light after being reflected by the surface of the ink layer in the dried state, the polarizing plate inserted in the light-receiving optical system blocks (prevents) the illumination light after being regularly reflected by the surface of the ink layer from entering the light-receiving optical system.
For example, in the following Patent Literature 1, as illustrated in FIG. 11, a measurement is performed using a polarizing plate 100 formed by bonding a circular ring-shaped first polarizing plate 101 and a circular-shaped second polarizing plate 102 disposed in a central region of the first polarizing plate 101 together in such a manner as to allow polarizing directions thereof to mutually intersect orthogonally. In the measurement, a reflection property of a sample surface s is measured under a condition that illumination light 103 is passed through the second polarizing plate 102, and the illumination light after being reflected by the sample surface s (reflected light 104) is passed through the first polarizing plate 101. In this manner, the first polarizing plate 101 and the second polarizing plate 102 whose polarizing directions mutually intersect orthogonally are arranged in adjacent relation, so that it becomes possible to arrange an illumination optical system and a light-receiving optical system in adjacent relation to achieve a reduction in size of a reflection property measuring device.
In the measurement of the reflection property of the sample surface s, in order to ensure sufficient measurement accuracy, it is necessary to allow the polarizing direction (polarizing axis) of the second polarizing plate 102 for passing the illumination light 103 therethrough to accurately intersect orthogonally with the polarizing direction (polarizing axis) of the first polarizing plate for passing therethrough the reflected light 104 as the illumination light after being reflected by the sample surface s 103. For example, the ISO Standard specifies that the accuracy should fall within ±5°.
However, the polarizing plates 101, 102 are arranged such that the small circular-shaped second polarizing plate 102 is disposed radially inside the first polarizing plate 101. Thus, during bonding between the first polarizing plate 101 and the second polarizing plate 102, it was difficult work to allow the respective polarizing directions thereof to accurately intersect orthogonally.